(1) Field of the Invention
This invention relates to a sintered electric contact material for vacuum switch tubes which maintains excellent withstand voltage performance even after a large number of load switching operations and has excellent circuit breaking performance, and to a process for manufacturing the same.
(2) Description of the Prior Art
Characteristic requirements of an electric contact material for use in vacuum switch tubes can be enumerated as excellent circuit breaking (current cutoff) performance, excellent withstand voltage performance, small chopping current, low material consumption, small tripping force against welding, low material transfer, etc., and there is a demand for a contact material which fulfills all these requirements. On the other hand, there are many cases in which the vacuum switch tube is exclusively used for an extremely large number of make-break cycles, for current closing or for current cutoff.
The conventional contact materials, in general, have a well-balanced combination of performances, but yet do not meet all the performance requirements. Therefore, the conventional contact materials are not satisfactorily suitable for use where a large number of current cutoff operations are to be performed or current closing operations. For instance, a Cu-W contact material has often been used in vacuum switch tubes for a current cutoff switch because of its excellent withstand voltage performance, but the withstand voltage performance is gradually lowered when the switch is frequently used for current closing operations. In addition, the Cu-W contact material is essentially low in breaking performance.
Thus, while the conventional contact materials for vacuum switch tubes have an overall well-balanced combination of performances, when applied to a use in which a specified kind of performance is of particular importance, the contact materials may fail to fulfill the requirement as to the performance characteristic. Accordingly, there is a demand for development of a new contact material.